In semiconductor devices, it is common to form a conductive structure that is buried within one or more insulating layers. The conductive structure is usually formed on a first dielectric layer (e.g., silicon dioxide) using conventional photolithographic patterning and etching. A second dielectric layer is then formed over the conductive structure. Electrical connection to the conductive structure is provided by a contact. The contact is typically formed by etching a contact window through the second dielectric layer to expose the conductive structure and, in a subsequent processing step, filling the contact window with a metal, such as aluminum. FIG. 1 is a cross-sectional view illustrating a standard lateral diffused metal-oxide-semiconductor (LDMOS) device 100 including a gate shield 102 which is formed in the manner described above.
One disadvantage of this conventional methodology of forming a buried conductive structure (e.g., gate shield 102) is that there is often a significant resistance associated with a junction (e.g., 106) between the contact (e.g., 104) and the conductive structure. This resistance, commonly referred to as contact resistance, is often greater than about twenty ohms, and can be as high as about 100 ohms. In a power semiconductor device, for example, wherein there may be a large current (e.g., milliamperes) passing through the contact, this contact resistance can significantly degrade device performance. It is therefore desirable to minimize contact resistance in a semiconductor device.
Previous attempts to minimize the contact resistance associated with a buried conductive structure have involved the use of polysilicon material for the conductive structure and contact. Polysilicon is easily patterned by conventional lithography and etch techniques, and can withstand high temperatures used in semiconductor processing. Unfortunately, however, polysilicon has a relatively high sheet resistance (e.g., typically about 30 to 200 ohms per square) which limits the conductivity of the conductive structure. Although it is known to form a silicide layer on the polysilicon in order to reduce the sheet resistance of the conductive structure, patterning a polysilicon layer that is reacted to form a silicide, such as titanium silicide, involves complicated processing steps and high temperature to form the silicide, thereby increasing the cost of fabrication and reducing yield. In alternative methodologies, aluminum has been used to form the conductive structure and the contact. Using aluminum to form the conductive structure, however, limits the temperatures at which the wafer can be subsequently processed, and is thus undesirable.
Accordingly, there exists a need for techniques for forming a buried conductive structure in a semiconductor device, and for providing electrical connection to the conductive structure, which does not suffer from one or more of the problems exhibited by conventional methodologies.